1. Field
The invention relates to rain detection systems for the control of automatic irrigation systems.
2. State of the Art
Automatic irrigation control systems are generally controlled primarily by a timing mechanism. There are systems which override the timing system when a certain quantity of rain is detected. This capability is desirable for both water conservation and to prevent damage to vegetation from over watering. Examples of such systems are disclosed in U.S. Pat. Nos. 4,613,764 and 3,297,254. Both of these patents describe systems wherein rainwater is collected in a collection tray. In each, two conductive sensors extend down into the collection tray to an adjustable depth from the bottom of the tray. An electronic switch is activated when the water level in the collection tray reaches the sensors, thereby electrically bridging the sensors.
In the prior art, the amount of rainfall which will interrupt the regular irrigation cycle is adjusted by adjusting the height of the sensors above the bottom of the rainwater-collection tray. The higher the bottom of the sensors are from the bottom of the tray, the more rainfall required to fill the tray to the level of the sensors. This method of adjustment limits the amount of rainfall which can be received without interrupting the irrigation cycle to approximately the depth of the rainwater-collection tray.
A further limitation of these systems is that debris which settles to the bottom of the rainwater collection tray reduces the effective depth of rain required to interrupt the regular cycle of the irrigation system. The amount of rainfall needed to interrupt the irrigation cycle will decline as debris builds up in the bottom of the rainwater-collection tray. Debris, such as wet leaves, can also make contact directly with the sensors and interrupt the regular irrigation cycle of the system. A method previously used to prevent this problem is to place a screen over the opening of the rainwater-collection tray. This method is disclosed, for example, in U.S. Pat. No. 3,212,714. This method has the disadvantage that the debris can build up on the screen and restrict the area of the opening of the tray. Thus, as debris builds up on the screen, less water from a given amount of rain will collect in the tray. In such instance, the amount of rainfall necessary to interrupt the irrigation cycle will increase as debris builds up on the screen.